1523 RESEARCH C old acclimation, or cold-hardening, occurs in many plant species in response to a period of low, above-freezing temperatures and short daylengths, as occur in the autumn in temperate regions of the world. Cold acclimation is associated with a plethora of metabolic changes that are thought to contribute to the ability to withstand subsequent freezing to potentially damaging tempera-). Specifically with wheat (Triticum aestivum L.), Herman et al. (2006) found that plants that had been cold-acclimated at +3°C for 3 wk, and then exposed to −3°C for 6 h to 3 d, showed an increase in freezing tolerance of more than 2°C, concomitant with changes in the mRNA levels of hundreds of genes, indicating the plants continued to acclimate while in the subzero environment.In a study of five winter wheat lines and their diallel progeny, the minimum temperature significantly impacted survival in 73% of the populations studied, while the length of time the plants were held at the minimum temperature was a significant factor in only 40% of the populations, and those were all progeny populations, not the parent lines (Skinner and Mackey, 2009). This observation suggested that time and temperature in the subzero environment impacted survival in a genotype-specific manner. Because the severity of cold experienced by plants in the field varies greatly from year to year and from place to place within a field (Fowler, 1979), the time and temperature combinations plants experience ABSTRACT To survive the temperature fluctuations that occur during the winter months, winter wheat (Triticum aestivum L.) plants must tolerate episodes of freezing of varying intensity and duration. In this study, the ability of six wheat cultivars to survive exposure to −13.5 to −16.5°C for 2, 3, or 4 h was measured. Tolerance was described as the percentages of plants surviving after time intervals at −10°C or less. The average survival of the six cultivars significantly decreased by about 50% as length of exposure to −10°C or less was increased from 325 to 525 min. Surprisingly, survival then increased with longer exposure times such that the percentages of plants surviving were similar after 675, and after 325 min at −10°C or less. The greater survival after longer exposure times to −10°C or less compared with shorter exposure times suggested that certain freezing tolerance mechanism(s) in wheat are activated only after sufficient exposure to subfreezing temperatures for a sufficient amount of time. Lesser exposure to either temperature or time apparently does not activate this mechanism, leading to greater plant death. Further characterization of this mechanism and its genetic control may lead to development of wheat lines with enhanced ability to survive harsh freezing conditions and thus improved winterhardiness.